Continuous hot-dip metal coating method and continuous hot-dip metal coating line

ABSTRACT

A continuous hot-dip metal coating method that can reduce both non-coating caused by metal vapor generated in a snout and non-coating caused by an oxide film on a molten metal bath surface in the snout and stably and promptly change the oxidizability of the atmosphere in the snout is provided. In a continuous hot-dip metal coating method, oxidizing gas is supplied into a snout  14 , a temperature of an inner wall surface of the snout is maintained at 150° C. or less below a temperature of the molten metal bath, and an atmospheric temperature of an upper portion in the snout is maintained at 100° C. or less below the temperature of the molten metal bath.

TECHNICAL FIELD

The disclosure relates to a continuous hot-dip metal coating method andcontinuous hot-dip metal coating line used to continuously manufacture,for example, a hot-dip galvanized steel sheet.

BACKGROUND

In a continuous hot-dip galvanizing line for steel strips, typically, asteel strip whose surface has been cleaned is continuously annealed inan annealing furnace, and cooled to a predetermined temperature. Thesteel strip is then entered into a molten zinc bath, to be hot-dipgalvanized. The annealing and cooling in the annealing furnace aretypically performed in a reducing atmosphere. To block the steel strippassage from the air and enable the steel strip to pass in the reducingatmosphere while the steel strip leaves the annealing furnace and entersthe molten zinc bath, a passage called a snout that is rectangular insection is provided between the annealing furnace and a coating tankcontaining the molten zinc bath. A sink roll is installed in the moltenzinc bath. Having entered the molten zinc bath, the steel strip changesits traveling direction by the sink roll, and moves upward. The steelstrip pulled up from the molten zinc bath is adjusted to have apredetermined coating thickness by gas wiping nozzles. After this, thesteel strip is cooled, and guided to subsequent steps.

The snout is connected to a cooling zone (steel strip delivery side) inthe annealing furnace, and so the inside of the snout normally has areducing atmosphere. This makes the formation of an oxide film on themolten zinc bath surface in the snout difficult, and only a thin oxidefilm forms. The oxide film formed on the molten zinc bath surface in thesnout is therefore not firm. Accordingly, when the steel strip entersthe molten zinc bath, molten zinc is exposed on the bath surface due tovibration or the like, and zinc evaporates from the bath surface intothe snout. In such a case, molten zinc evaporates to the saturationvapor pressure at the atmospheric temperature in the snout.

Zinc vapor reacts with oxygen present in a very small amount in thereducing atmosphere gas, to form an oxide. Even in the case where zincvapor is not oxidized, when the vapor pressure of zinc vapor reaches thesaturation vapor pressure or more, part of zinc vapor phase-changes tozinc in the liquid phase or the solid phase. In particular, since thesnout is merely made of a thin heat-resistant material, the temperatureof the inner wall surface of the snout tends to be less than or equal tothe saturation temperature at the vapor pressure of zinc vapor due tothe influence of external air. In a site where the temperature is lessthan or equal to the saturation temperature, zinc vapor becomes zincpowder and adheres to the inner surface of the snout.

If such oxide or deposit (ash) adheres to the steel strip, qualitydefects such as non-coating portions occur. Quality defects such asnon-coating portions caused by ash generated due to zinc vapor in thesnout are hereafter referred to as “ash-caused defects”.

The following techniques have been proposed to reduce ash-causeddefects. JP H8-176773 A (PTL 1) describes a technique of heating thesnout with a heater and insulating the outside of the heater with a heatinsulator so that the temperature difference between the molten bathtemperature and each of the atmospheric temperature and inner walltemperature in the snout is 150° C. or less, thus preventing ashadhesion to the snout inner wall. JP H8-302453 A (PTL 2) describes atechnique of installing a suction blower in the molten bath andconnecting, to the suction side of the suction blower, a suction tubethat has a suction port at a position higher than the bath surface inthe snout, to discharge zinc vapor in the snout to outside the system.JP H6-330271 A (PTL 3) describes a technique of setting the atmospherein the snout to non-oxidizing gas for a steel sheet and to oxidizing gasfor molten zinc to suppress the generation of fumes (zinc vapor).

CITATION LIST Patent Literatures

PTL 1: JP H8-176773 A

PTL 2: JP H8-302453 A

PTL 3: JP H6-330271 A

SUMMARY Technical Problem

The technique in PTL 1 can reduce the crystallization of zinc vapor onthe snout inner wall, that is, the generation of ash, to some extent byheating the snout. However, the generation of zinc vapor from the moltenzinc bath surface itself cannot be prevented, and so ash is inevitablygenerated in a site that is not heated. The technique thus cannoteliminate a potential risk of ash adhering to the steel strip.

With the technique in PTL 2, zinc vapor in the snout cannot bedischarged completely. Zinc vapor that has not been discharged adheresto the snout inner wall, resulting in the generation of ash. The effectof preventing ash-caused defects is thus insufficient. Besides,discharging zinc vapor rather facilitates the evaporation of moltenzinc, and therefore tends to be not effective.

With the technique in PTL 3, since there is considerably rapid gasconvection in the snout, most of the supplied oxidizing gas is releasedfrom the system without staying on the bath surface. Unless an extremelylarge amount of gas is supplied, an appropriate oxide film cannot beformed, and it is difficult to prevent the evaporation of molten zinc.

Thus, the effect of reducing ash-caused defects by each of thetechniques in PTL 1 to PTL 3 is insufficient. Moreover, our studyrevealed that, in the case where the oxide film is excessively thick,the oxide film adheres to the steel strip surface when the steel stripenters the molten zinc bath, and causes quality defects such asnon-coating portions. Quality defects such as non-coating portionscaused by the oxide film on the molten zinc bath surface in the snoutare hereafter referred to as “oxide film-caused defects”.

The techniques in PTL 1 to PTL 3 also have the following problem. Thesuitable oxidizability of the atmosphere in the snout (especially nearthe bath surface) varies depending on an operation condition such as thechemical composition of the steel strip, the annealing condition in theannealing, or the component of the molten metal bath. Accordingly, whenthe operation condition is switched over, the oxidizability of theatmosphere in the snout needs to be changed promptly. With thetechniques in PTL 1 to PTL 3, however, the oxidizability of theatmosphere in the snout cannot be changed stably and promptly.Particularly in PTL 3, the presence of large natural convection in thesnout makes it impossible to stably and promptly change theoxidizability of the atmosphere in the snout.

These problems are not limited to hot-dip galvanizing, but apply tohot-dip metal coating in general.

It could be helpful to provide a continuous hot-dip metal coating methodand continuous hot-dip metal coating line that can reduce bothnon-coating caused by metal vapor generated in a snout and non-coatingcaused by an oxide film on a molten metal bath surface in the snout andstably and promptly change the oxidizability of the atmosphere in thesnout.

Solution to Problem

As a result of careful examination, we discovered the following.

(A) To reduce ash-caused defects by suppressing the evaporation ofmolten zinc (the generation of zinc vapor), an oxide film with at leasta predetermined thickness needs to be formed on the bath surface. Toreduce oxide film-caused defects, on the other hand, the oxide filmthickness needs to be limited to a predetermined thickness or less. Thismeans an oxide film with an optimum thickness needs to be formed inorder to reduce both ash-caused defects and oxide film-caused defects.

(B) To form an oxide film with an optimum thickness, there is a need toprecisely manage the dew point of the atmosphere near the molten zincbath surface in the snout by, while suppressing the convection of theatmosphere in the snout, supplying oxidizing gas into the snout. Thebest way to achieve this is to supply minimum necessary oxidizing gasinto the snout in a state where the heat convection of the atmosphere inthe snout is suppressed. This enables oxidizing gas supplied near thebath surface to substantially stay near the bath surface.

(C) Consequently, the oxidizability of the atmosphere in the snout canbe changed stably and promptly. Hence, when the operation condition isswitched over or changed, the oxidizability of the atmosphere in thesnout can be promptly changed according to the changed operationcondition.

The disclosure is based on these discoveries. We thus provide:

(1) A continuous hot-dip metal coating method comprising: continuouslyannealing a steel strip in an annealing furnace; and continuouslysupplying the steel strip after the annealing into a coating tankcontaining a bath of molten metal, to metal-coat the steel strip,wherein while the steel strip traveling from the annealing furnace tothe molten metal bath passes through a space defined by a snout that islocated on a steel strip delivery side of the annealing furnace and hasan end immersed in the molten metal bath, oxidizing gas is supplied intothe snout, a temperature of an inner wall surface of the snout ismaintained at 150° C. or less below a temperature of the molten metalbath, and an atmospheric temperature of an upper portion in the snout ismaintained at 100° C. or less below the temperature of the molten metalbath.

(2) The continuous hot-dip metal coating method according to (1),wherein the oxidizing gas comprises any one of nitrogen gas containingwater vapor and nitrogen-hydrogen mixed gas containing water vapor.

(3) The continuous hot-dip metal coating method according to (1),wherein oxidizability of the oxidizing gas is changed depending on anoperation condition.

(4) The continuous hot-dip metal coating method according to (2),wherein an amount of the water vapor in the oxidizing gas is changeddepending on an operation condition.

(5) The continuous hot-dip metal coating method according to (2),further comprising preliminarily investigating, for each operationcondition, a relationship between a dew point in the snout and an amountof a defect caused by non-coating of the steel strip metal-coated underthe operation condition, to determine a target dew point in the snoutunder the operation condition, wherein an amount of the water vapor inthe oxidizing gas is determined based on the target dew point determinedfor the each operation condition.

(6) The continuous hot-dip metal coating method according to (5),wherein when the operation condition is switched over, the amount of thewater vapor in the oxidizing gas is changed based on the target dewpoint corresponding to the changed operation condition.

(7) The continuous hot-dip metal coating method according to any one of(3) to (6), wherein the operation condition is at least one of achemical composition of the steel strip, an annealing condition in theannealing, and a component of the molten metal bath.

(8) The continuous hot-dip metal coating method according to any one of(3) to (6), wherein the operation condition is a chemical composition ofthe steel strip.

(9) The continuous hot-dip metal coating method according to any one of(1) to (8), wherein the oxidizing gas is supplied from both edges of thesnout in a transverse direction of the steel strip.

(10) A continuous hot-dip metal coating line comprising: an annealingfurnace that continuously anneals a steel strip; a coating tankcontaining a bath of molten metal; a snout located on a steel stripdelivery side of the annealing furnace, having an end immersed in themolten metal bath, and defining a space through which the steel stripcontinuously supplied from the annealing furnace into the molten metalbath passes; a heating unit provided on an outer wall of the snout andin an upper portion in the snout; a gas supply mechanism connected tothe snout; and a controller that controls the heating unit and the gassupply mechanism to supply oxidizing gas into the snout, maintain atemperature of an inner wall surface of the snout as 150° C. or lessbelow a temperature of the molten metal bath, and maintain anatmospheric temperature of an upper portion in the snout at 100° C. orless below the temperature of the molten metal bath.

Advantageous Effect

It is possible to provide a continuous hot-dip metal coating method andcontinuous hot-dip metal coating line that can reduce both non-coatingcaused by metal vapor generated in a snout and non-coating caused by anoxide film on a molten metal bath surface in the snout and stably andpromptly change the oxidizability of the atmosphere in the snout.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a schematic view of a continuous hot-dip galvanizing line 100according to one of the disclosed embodiments;

FIG. 2 is a view illustrating half of the inside of a snout 14 in FIG. 1from the transverse center of a steel strip P;

FIG. 3 is an enlarged schematic view of the snout 14 in FIG. 1;

FIG. 4 is a graph illustrating the relationship between theoxidizability of the bath surface atmosphere and the defect rate;

FIG. 5A is a graph illustrating the relationship between theoxidizability of the bath surface atmosphere and the defect rate forhigh Si-containing steel and low Si-containing steel;

FIG. 5B is a graph illustrating the relationship between theoxidizability of the bath surface atmosphere and the oxide filmthickness for a high Al-containing bath and a low Al-containing bath;

FIG. 6A is a graph illustrating the relationship between the dew pointin the snout and the defect rate for steel type A;

FIG. 6B is a graph illustrating the relationship between the dew pointin the snout and the defect rate for steel type B; and

FIG. 7 is a graph illustrating the change of the dew point in the snoutin each of Examples 1 to 3 and Comparative Examples 1 and 2.

DETAILED DESCRIPTION

A continuous hot-dip galvanizing line 100 and a continuous hot-dipgalvanizing method using the continuous hot-dip galvanizing line 100according to one of the disclosed embodiments are described below.

In FIG. 1, the continuous hot-dip galvanizing line 100 includes anannealing furnace 10, a coating tank 12, and a snout 14.

The annealing furnace 10 is a device that continuously anneals a steelstrip P passing through the annealing furnace 10, and includes a heatingzone, a soaking zone, and a cooling zone arranged side by side in thisorder. Only the cooling zone is illustrated in FIG. 1. The annealingfurnace may have a well-known structure or any structure. Reducing gasor non-oxidizing gas is typically supplied into the annealing furnace.As the reducing gas, H₂—N₂ mixed gas is typically used. An example ofsuch gas is gas (dew point: about −60° C.) having a compositioncontaining H₂: 1 vol % to 20 vol % with the balance being N₂ andincidental impurities. An example of the non-oxidizing gas is gas (dewpoint: about −60° C.) having a composition containing N₂ and incidentalimpurities. The annealed steel strip P is cooled to about 470° C. to500° C. in the cooling zone.

The coating tank 12 contains a molten zinc bath 12A. The snout 14 islocated on the steel strip delivery side of the annealing furnace 10.The snout 14 is connected to the cooling zone in this embodiment. Asnout end 14A is immersed in the molten zinc bath 12A. The snout 14 is amember that defines the space through which the steel strip Pcontinuously supplied from the annealing furnace 10 into the molten zincbath 12A passes. A turndown roll 26 for changing the traveling directionof the steel strip P from horizontal to obliquely downward is located inan upper portion in the snout 14. The part of the snout 14 that definesthe space through which the steel strip P having passed through theturndown roll 26 passes is rectangular in section perpendicular to thetraveling direction of the steel strip P.

The steel strip P passes through the inside of the snout 14, andcontinuously enters the molten zinc bath 12A. A sink roll 28 and supportrolls 30 are installed in the molten zinc bath 12A. Having entered themolten zinc bath 12A, the steel strip P is changed upward in the sheetpassing direction by the sink roll 28, and then guided by the supportrolls 30 to leave the molten zinc bath 12A. The steel strip P is thushot-dip galvanized.

In FIG. 2, the continuous hot-dip galvanizing line 100 includes a gassupply mechanism 20 connected to the snout 14. The gas supply mechanism20 includes: a first pipe 22A through which hydrogen gas passes; asecond pipe 22B through which nitrogen gas passes; a third pipe 22Cthrough which water vapor as oxidizing gas passes; flow rate adjustingvalves 24 attached to these pipes; a fourth pipe 22D through which mixedgas obtained by mixing the gases supplied from these pipes passes; and afifth pipe 22E connected to the fourth pipe 22D and has its tip locatedinside the snout 14. The first pipe 22A and the third pipe 22C areconnected to the second pipe 22B. By regulating the valves 24, hydrogen,nitrogen, and water vapor can be mixed at any flow ratio.

The oxidizing gas is not limited, and may be gas containing water vapor,oxygen, carbon dioxide, or the like. Gas containing water vapor ispreferable because its oxidizability is not excessively high and so itis easy to be managed, is inexpensive, and is easy to be measured inoxidizability by a dew point meter.

In FIG. 3, a heater 16 as a heating unit is placed on the outer wall ofthe snout 14, and covered with a heat insulator 18. The heater 16 coversthe whole outer wall except the tip portion of the snout 14 (near thebath surface). A heater 17 as a heating unit is also placed in the upperportion in the snout. Since the upper portion in the snout hassignificant influence on the occurrence of heat convection as describedlater, providing the heater 17 ensures that the atmospheric temperatureof the upper portion in the snout is increased.

In this embodiment, it is important that a controller (not illustrated)controls the heaters 16 and 17 and the gas supply mechanism 20 to supplythe oxidizing gas into the snout 14 and maintain the temperature of theinner wall surface of the snout 14 at (a temperature of the molten metalbath−150° C.) or more and the atmospheric temperature of the upperportion in the snout 14 at (the temperature of the molten metalbath−100° C.) or more. The technical significance of this control isdescribed below.

As mentioned earlier, there is an optimum level for the oxidizability ofthe atmosphere in the snout. FIG. 4 illustrates the concept of such anoptimum level. In the case where the oxidizability is low, no oxide filmforms on the bath surface or, even when an oxide film forms, the oxidefilm is very thin. In this case, oxide film-caused defects are unlikelyto occur, but zinc evaporates actively and so ash-caused defectsincrease. In the case where the oxidizability is high, a thick oxidefilm serves as a protective film and zinc hardly evaporates. In thiscase, ash-caused defects are unlikely to occur, but oxide film-causeddefects occur a lot.

It is therefore necessary to precisely control the oxidizability of theatmosphere near the bath surface where zinc evaporates or oxidizes, tothe optimum level (the center part in FIG. 4). We discovered that, forexample in the case of controlling the oxidizability of the atmospherenear the bath surface by supplying gas containing water vapor into thesnout, both ash-caused defects and oxide film-caused defects can bereduced to low level by precisely controlling the dew point of theatmosphere near the bath surface within the range of about apredetermined point (target dew point) ±4° C. The target dew point canbe determined by the below-mentioned method once the operationconditions other than the target dew point are determined.

Here, the convection of the atmosphere in the snout makes it difficultto manage the dew point near the bath surface. Main convection in thesnout includes an accompanying flow that occurs due to the movement ofthe steel strip, a heat convection flow associated with the temperaturedifference in the snout, and a pressure flow caused by the pressuredifference in the snout. Under a normal snout condition, the influenceof heat convection flow is dominant. For example, in the case where thesteel strip temperature is 500° C. and the temperature of the moltenmetal bath is 450° C., the temperature difference of the inside of thesnout from the outside of the snout is 400° C. or more. Moreover, sincethe upper portion in the snout is usually connected to the cooling zone,the atmospheric temperature of the upper portion in the snout tends tobe 200° C. to 300° C. In such a case, the wind velocity by heatconvection is about 4 m/s to 5 m/s, which is considerably higher than atypical value of a steel strip accompanying flow of 1 m/s.

Even when gas facilitating bath surface oxidation, such as gascontaining water vapor, is supplied in this situation, most of the gasdoes not stay on the bath surface. To form an oxide film with anappropriate thickness for reducing ash-caused defects, a large amount ofwater vapor needs to be supplied. Meanwhile, to reduce oxide film-causeddefects, the concentration distribution of the oxidizing gas near thebath surface needs to be minimized because a thinner oxide film is moreadvantageous. Under a large heat convection condition, however, theconcentration distribution of the oxidizing gas near the bath surface ishigh (i.e. the concentration is not uniform within the surface), so thatmanaging the dew point near the bath surface is extremely difficult.

Based on the aforementioned discoveries, we concluded that the mosteffective way of precisely managing the dew point near the bath surfaceto reduce both ash-caused defects and oxide film-caused defects is tosuppress zinc evaporation, and the best way of suppressing zincevaporation is to supply minimum necessary oxidizing gas into the snoutwhile suppressing heat convection in the snout.

We then aimed to reduce the temperature difference in the snout whichcauses such heat convection. Although the steel strip is highest intemperature in the snout, the steel strip temperature is normally higherthan the bath temperature only by about 10° C. Hence, the temperature ofthe molten metal bath is used as the reference temperature in thedisclosure. Since the heat convection flow and the steel stripaccompanying flow are in opposite directions, the convection in thesnout is greatly reduced if the magnitude of the heat convection flowcan be limited to not more than twice the magnitude of the steel stripaccompanying flow.

As a result of careful examination, we discovered that, by maintainingthe temperature of the inner wall surface of the snout at (thetemperature of the molten metal bath−150° C.) or more, the convection ofthe atmosphere in the snout can be reduced to such a flow state wherethe influence of temperature is negligible. Here, the atmospherictemperature of the upper portion in the snout has more influence on heatconvection, and so needs to be maintained at (the temperature of themolten metal bath−100° C.) or more. This is because a density flow hashigher flow velocity in the case where gas having high density ispresent at a high position. (A flow caused by density is proportional toΔρgh where h is the height difference. The presence of a high densitysubstance at a high position increases flow velocity.)

The atmospheric temperature of the upper portion in the snout ispreferably (the temperature of the molten metal bath+100° C.) or less.Although the convection in the snout is more stabilized when theatmospheric temperature of the upper portion is higher (the presence ofa low density substance in the upper portion contributes to a stablestate), the stabilizing effect is saturated if the atmospherictemperature of the upper portion is more than (the temperature of themolten metal bath+100° C.). The temperature of the inner wall surface ofthe snout is preferably (the temperature of the molten metal bath+0° C.)or less. If the temperature of the inner wall surface is higher than thetemperature of the molten metal bath, an upward flow occurs near theside wall in the snout, as a result of which a downward flow occurs inthe center portion. Since this flow is in the same direction as thesteel strip accompanying flow, a large flow will result in the snout.Thus, there is no need to maintain the temperature of the inner wallsurface at higher than the temperature of the molten metal bath, andrather such temperature control is likely to cause a larger flow.

The term “upper portion in the snout” in the disclosure means the regionin the snout within 1 m from the surface of the turndown roll. In FIG.3, the upper portion in the snout is the region within 1 m from thesurface of the turndown roll 26 in the snout 14.

By supplying the oxidizing gas into the snout in a state where thetemperature of the inner wall surface of the snout and the atmospherictemperature of the upper portion in the snout are managed in this way,most of the oxidizing gas reaching near the bath surface can stay on thebath surface, so that the generation of zinc vapor can be suppressedwith a smaller amount of gas. Moreover, since the gas component suppliedinto the snout is present near the bath surface with substantially nochange, the atmosphere can be controlled easily, with it being possibleto reduce the variation of the dew point of the atmosphere near the bathsurface. Consequently, oxide film-caused defects can be reduced, too.Thus, the oxidation state of the bath surface in the snout can bemaintained ideally, so that both ash-caused defects and oxidefilm-caused defects can be almost eliminated. Further, the oxidizabilityof the atmosphere in the snout can be changed stably and promptly.Hence, when an operation condition is switched over, the oxidizabilityof the atmosphere in the snout can be promptly changed according to thechanged operation condition.

The oxidizing gas supplied into the snout is preferably nitrogen gascontaining water vapor or nitrogen-hydrogen mixed gas containing watervapor. The dew point of the oxidizing gas may be set as appropriatedepending on the composition of the molten bath, the steel type to bemanufactured, and other operation conditions, but tends to be favorablein the range of about −20° C. to −35° C. Although the oxidizing gassupply amount depends on various operation conditions, in the case wherethe conditions other than the temperature of the inner wall surface ofthe snout and the atmospheric temperature of the upper portion in thesnout are the same, the same dew point can be achieved with a supplyamount of about ¼ as compared with when the temperature of the innerwall surface and the atmospheric temperature of the upper portion areoutside the ranges according to the disclosure. The oxidizing gas supplyamount can thus be reduced to the minimum necessary amount for formingan appropriate oxide film.

As illustrated in FIG. 2, the oxidizing gas is preferably supplied intothe snout 14 from both edges of the snout in the steel strip transverse(width) direction. The reason why the fifth pipe 22E having a gas supplyport is located on the side surface of the snout 12 is that, since thetemperature near the side surface in the snout tends to be low and so adownward flow usually occurs near the side surface, the oxidizing gascan be efficiently delivered to near the bath surface. The height of thegas supply port from the bath surface may be about 100 mm to 3000 mm. Ifthe height is less than 100 mm, the gas is highly likely to directlyreach the bath surface, causing an increase in concentrationdistribution of the oxidizing gas near the bath surface. If the heightis more than 3000 mm, the gas concentration decreases due to a longdistance from the bath surface, so that a large amount of gas is needed.

The suitable oxidizability of the atmosphere near the bath surface inthe snout varies depending on an operation condition such as thechemical composition of the steel strip, the annealing condition in theannealing, or the component of the molten zinc bath. In other words, thetwo curves illustrated in FIG. 4 can shift right or left depending onthe operation condition. This is described below, with reference toFIGS. 5A and 5B as an example.

Both ash-caused defects and oxide film-caused defects correlate to thethickness of the oxide film formed on the bath surface, as mentionedabove. In detail, ash-caused defects relate to the amount of ash and itsadhesion rate, and oxide film-caused defects relate to the amount ofoxide film and its adhesion rate.

FIG. 5A illustrates an example of the influence of the chemicalcomposition of the steel strip on the suitable oxidizability of theatmosphere near the bath surface in the snout. In the case where thesteel strip contains a lot of oxidizable element such as Si, Mn, or Al,a large amount of oxide is concentrated on the surface of the steelstrip immediately before entering the molten bath. If the steel strip iscoated in such a surface concentration state, the oxide film easilyadheres to the steel strip, that is, the adhesion rate of the oxide filmis high, facilitating oxide film-caused defects. On the other hand, theamount of ash hardly depends on the surface concentration state of thesteel strip, and so the chemical composition of the steel strip hardlyinfluences ash-caused defects.

The surface concentration state of the steel strip also differsdepending on the annealing condition such as the annealing temperatureand the furnace dew point. Thus, the annealing condition also influencesoxide film-caused defects, but hardly influences ash-caused defects.

FIG. 5B illustrates an example of the influence of the component of themolten zinc bath on the suitable oxidizability of the atmosphere nearthe bath surface in the snout. As illustrated in FIG. 5B, when the Alconcentration in the bath is higher, an oxide film forms on the bathsurface more easily. Thus, a high Al-containing bath causes fewerash-caused defects, and more oxide film-caused defects. In other words,the two curves in FIG. 4 shift to the left.

It is therefore preferable to change the oxidizability of the oxidizinggas depending on the operation condition. In detail, in the case wherethe oxidizing gas contains water vapor, the amount of water vapor in theoxidizing gas is changed depending on the operation condition, as thesuitable dew point of the atmosphere near the bath surface, i.e. thetarget dew point, differs depending on the operation condition. Theamount of water vapor in the oxidizing gas is typically 100 ppm or more.

In this case, for each operation condition, the relationship between thedew point in the snout and the defect rates of ash-caused defects andoxide film-caused defects (i.e. the information in FIG. 4) may bepreliminarily investigated to determine the target dew point in thesnout under the operation condition. The amount of water vapor in theoxidizing gas may then be determined based on the target dew pointdetermined for the operation condition. When the operation condition isswitched over, the amount of water vapor in the oxidizing gas may bechanged based on the target dew point corresponding to the changedoperation condition.

The relationship between the dew point in the snout and the defect ratesof ash-caused defects and oxide film-caused defects as illustrated inFIG. 4 can be determined by preliminarily recognizing the tendency ofthe correspondence between the dew point in the snout and the defectrate of each type of defect in past operation. Whether or not each typeof defect occurs may be visually determined. The size of a visuallyobservable defect is about 100 μm or more. The rate of defect occurrenceper 0.5 m in length is defined as “defect rate”. A defect rate of 1%means one defect per 50 m.

The aforementioned dew point in the snout needs to be the dew pointimmediately above the bath surface (near the bath surface). In the casewhere the actual dew point measurement location is not immediately abovethe bath surface, the following adjustment is performed. In a statewhere heat convection in the snout is eliminated according to thedisclosure, there is hardly any dew point distribution in the snout, andso the actual measured dew point can be directly used as the dew pointin the snout. If there is heat convection in the snout, however, theactual measured dew point is corrected to the dew point near the bathsurface. This correction can be performed using the dew pointdistribution predicted from flow analysis. For example, in the casewhere the dew point at a height of 500 mm from the bath surface is −35°C. and the dew point near the bath surface is −30° C. according to flowanalysis, the difference in dew point is +5° C., and the difference inwater ratio is 150 ppm. Accordingly, the dew point obtained by addingthe value corresponding to 150 ppm to the actual measured dew pointvalue at a height of 500 mm can be used as the bath surface dew point.

Examples of the operation condition influencing the suitableoxidizability of the atmosphere near the bath surface in the snout (thetarget dew point of the atmosphere near the bath surface in the casewhere the oxidizing gas contains water vapor) include the steel type(the chemical composition of the steel strip), the annealing conditionin the annealing, and the component of the molten zinc bath. At leastone of these operation conditions is preferably used to obtain theinformation in FIG. 4 beforehand. For example, in the case where it isknown that the annealing condition and the component of the molten zincbath are fixed in a specific continuous hot-dip galvanizing line, theinformation in FIG. 4 is preliminarily investigated for each steel typescheduled to pass through the line, to determine the target dew point.When the steel type is switched over, the amount of water vapor in theoxidizing gas is changed so that the target dew point corresponds to thechanged steel type.

The disclosure is not limited to the foregoing embodiment, and equallyapplies to the case of continuously hot-dip metal coating a steel strip.

EXAMPLES First Example

Using the continuous hot-dip galvanizing line in FIGS. 1 to 3, eachsteel strip (hereafter referred to as “steel type A”) having a chemicalcomposition containing, in mass %, C: 0.001%, Si: 0.01%, Mn: 0.1%, P:0.003%, S: 0.005%, and Al: 0.03% with the balance being Fe andincidental impurities and having a sheet thickness of 0.6 mm to 1.2 mm,a sheet width of 900 mm to 1250 mm, and a tensile strength of 270 MPawas entered into the molten zinc bath at a sheet passing speed of 60 mpmto 100 mpm, to manufacture a hot-dip galvanized steel sheet. The fifthpipe having a gas supply port was located on the side surface of thesnout, and the height of the gas supply port from the bath surface was500 mm, as illustrated in FIG. 2. The relationship between the dew pointin the snout and the defect rates of ash-caused defects and oxidefilm-caused defects was preliminarily investigated from past operationdata. FIG. 6A illustrates the results. Based on FIG. 6A, the target dewpoint in the snout was determined to be −30° C. This indicates that bothash-caused defects and oxide film-caused defects are reduced to lowlevel if the dew point in the snout can be controlled within the rangeof about −30° C.±4° C.

While the steel strip passed through the snout, nitrogen-hydrogen mixedgas containing water vapor was supplied from the gas supply port in testexamples No. 1 to 5 (“water vapor: supplied” in Table 1), andnitrogen-hydrogen mixed gas not containing water vapor was supplied fromthe gas supply port in test examples No. 6 and 7 (“water vapor: notsupplied” in Table 1). The dew point of the supplied gas in testexamples No. 1 to 5 was measured by a dew point meter provided in a dewpoint measurement hole 32A in the fifth pipe in FIG. 2, and is listed inTable 1.

The temperature of the snout inner wall surface and the atmospherictemperature of the upper portion in the snout while the steel strippassed through the snout were managed as listed in Table 1. In testexample No. 6, no heating by the heaters provided on the snout outerwall and in the upper portion in the snout was performed.

In each test example, the dew point of the atmosphere in the snout wasmeasured over time, by a dew point meter provided in a dew pointmeasurement hole 32B at a height of 500 mm in the center portion of theback of the snout in FIG. 2. In each of test examples No. 1 to 7, basedon the difference between the measured dew point and the target dewpoint (−30° C.), the flow rate of the supplied gas was changed so thatthe measured dew point was closer to the target dew point. This controlwas performed by typical PID control logic. A histogram of the measureddew point in each of test examples No. 1 to 7 is listed in Table 2. Foreach of test examples No. 1 to 5, the proportion of the volume of watervapor to the total volume of the supplied gas in the test is indicatedas “water amount” in Table 1, and the total gas supply flow rate in thetest is indicated as an index in Table 1 with the total flow rate of No.5 being set to 1.

Given that the dew point to be managed is the dew point immediatelyabove the bath surface, the dew point meter needs to be at a lowerposition near the bath surface. According to the disclosure, however,there is hardly any dew point distribution in the snout, so that the dewpoint near the bath surface can be accurately determined even when thedew point measurement is performed at a height of 500 mm. In ComparativeExamples with the generation of zinc vapor, the dew point meter cannotbe installed in the lower portion of the snout due to the risk of zincvapor adhering to the sensor part of the dew point meter if the dewpoint meter is at a low position such as a height of about 100 mm fromthe bath surface. While the gas measuring instrument was the dew pointmeter in this example as water vapor was used in the oxidizing gas, inthe case of using oxidizing gas other than water vapor, a measuringinstrument for detecting such gas needs to be installed.

(Evaluation of Defect Rate)

The defect rate of each of ash-caused defects and oxide film-causeddefects was evaluated by the following method. Whether or not each typeof defect occurred was visually determined. The size of a visuallyobservable defect is about 100 μm or more. The rate of defect occurrenceper 0.5 m in length is defined as “defect rate”, and listed in Table 1.A defect rate of 1% means one defect per 50 m.

(Evaluation Results)

The evaluation results are described below, with reference to Tables 1and 2. No. 1 (Example) is an example with no temperature differenceamong the bath temperature, the wall surface temperature, and the upperportion temperature. There was little variation in dew point, and as aresult ash-caused defects and oxide film-caused defects hardly occurred.No. 2 (Example) is an example with a low wall surface temperature, andNo. 3 (Example) is an example with a low atmospheric temperature of thesnout upper portion. In these examples, the dew point of the atmospherein the snout was able to be controlled within the management range (−30°C.±4° C.), as a result of which each defect rate was kept at low level.Moreover, in No. 1 to 3, the gas supply flow rate was sufficientlyreduced as compared with No. 5.

No. 4 (Comparative Example) is an example with the wall surfacetemperature being outside the range according to the disclosure, and No.5 (Comparative Example) is an example with the atmospheric temperatureof the snout upper portion being outside the range according to thedisclosure. In these examples, the dew point of the atmosphere in thesnout was unable to be controlled within the management range (−30°C.±4° C.), as a result of which many ash-caused defects or oxidefilm-caused defects occurred. No. 6 (Comparative Example) is an examplewithout water vapor supply and without heating by the heaters. In thiscase, the dew point was low around −40° C. and so oxide film-causeddefects did not occur, but a large number of ash-caused defectsoccurred. In No. 7 (Comparative Example), the dew point was stablebecause there was no temperature difference, but was low around −40° C.,so that a large number of ash-caused defects occurred.

TABLE 1 Bath Wall surface Upper portion Supply dew Water Supply flowDefect rate (%) temperature temperature temperature point amount rateAsh-caused Oxide film- No. Water vapor (° C.) (° C.) (° C.) (° C.) (ppm)(Nm³/hr) defect caused defect Category 1 Supplied 450 450 450 −29 4150.31 0.02 0.00 Example 1 2 Supplied 450 300 450 −28 460 0.29 0.06 0.03Example 2 3 Supplied 450 450 350 −28 460 0.36 0.05 0.03 Example 3 4Supplied 450 250 450 −20 1015 0.70 0.13 1.06 Comparative Example 1 5Supplied 450 450 300 −25 622 1 1.22 0.28 Comparative Example 2 6 Notsupplied 450 250 200 — — — 5.68 0.00 Comparative Example 3 7 Notsupplied 450 450 450 — — — 5.13 0.00 Comparative Example 4

TABLE 2 Measured dew point (° C.) No. 1 No. 2 No. 3 No. 4 No. 5 No. 6No. 7 to −20 1.1% −20 to −22 2.8% −22 to −24 6.7% −24 to −26 12.3% 3.0%−26 to −28 3.2% 11.2% 12.3% 17.3% 12.6% −28 to −30 49.6% 35.8% 40.0%21.6% 21.8% −30 to −32 44.3% 39.3% 37.1% 23.3% 20.6% −32 to −34 2.9%13.7% 10.6% 11.5% 18.6% −34 to −36 3.4% 12.5% 2.3% −36 to −38 6.8% 3.6%−38 to −40 3.3% 43.6% 63.8% from −40 0.8% 50.5% 36.2% Total 100.0%100.0% 100.0% 100.0% 100.0% 100.0% 100.0% Category Example ExampleExample Comparative Comparative Comparative Comparative Example ExampleExample Example

Second Example

The relationship between the dew point in the snout and the defect ratesof ash-caused defects and oxide film-caused defects was determined inthe same way as in the first example, except that, instead of the steelstrip of steel type A, each steel strip (hereafter referred to as “steeltype B”) having a chemical composition containing, in mass %, C: 0.12%,Si: 1.0%, Mn: 1.7%, P: 0.006%, S: 0.006%, and Al: 0.03% with the balancebeing Fe and incidental impurities and having a sheet thickness of 0.6mm to 1.2 mm, a sheet width of 900 mm to 1250 mm, and a tensile strengthof 780 MPa was used. FIG. 6B illustrates the results.

As illustrated in FIGS. 6A and 6B, for both steel types A and B, thereis a dew point at which both ash-caused defects and oxide film-causeddefects can be sufficiently reduced. In steel type B, however, theoptimum value, i.e. the target dew point, is lower, and the dew pointrange in which both defect rates are sufficiently low is narrower. Thisindicates that, for example when switching from steel type A to steeltype B, the dew point of the atmosphere needs to be changed accuratelyin a short time.

Third Example

The speed of changing the dew point of nitrogen-hydrogen mixed gascontaining water vapor was examined, in a state of the bath temperature,the wall surface temperature, and the upper portion temperature in No. 1to 5 (Examples 1 to 3 and Comparative Examples 1 and 2) in Table 1. Asillustrated in FIG. 7, the supply dew point was changed from −35° C. to−20° C. at 50 minutes.

In Example 1, the bath temperature, the wall surface temperature, andthe upper portion temperature were all set to 450° C., and so there washardly any heat convection. Accordingly, the measured dew point changedsubstantially in the same way as the change of the dew point of thesupplied gas. The dew point in the snout can thus be directly controlledusing the dew point of the supplied gas, which is very advantageous interms of quality management. In Examples 2 and 3, the changed dew pointhad some delay as compared with Example 1, but was able to follow thesupply dew point after about 30 minutes, which is sufficient in terms ofquality management.

In Comparative Examples 1 and 2, after the supply dew point was changed,the dew point in the snout continued to increase gradually whilevarying, and was far from being stable even after 1 hour. In such astate, it is difficult to respond to the change of the target dew pointwhen, for example, switching from steel type A to steel type B.

INDUSTRIAL APPLICABILITY

The disclosed continuous hot-dip metal coating method and continuoushot-dip metal coating line can reduce both non-coating caused by metalvapor generated in a snout and non-coating caused by an oxide film on amolten metal bath surface in the snout.

REFERENCE SIGNS LIST

-   -   100 continuous hot-dip galvanizing line    -   10 annealing furnace    -   12 coating tank    -   12A molten zinc bath    -   14 snout    -   14A snout end    -   16, 17 heater    -   18 heat insulator    -   20 gas supply mechanism    -   22A, 22B, 22C, 22D, 22E pipe    -   24 valve    -   26 turndown roll    -   28 sink roll    -   30 support roll    -   32A, 32B dew point measurement hole    -   P steel strip

1. A continuous hot-dip metal coating method comprising: continuouslyannealing a steel strip in an annealing furnace; and continuouslysupplying the steel strip after the annealing into a coating tankcontaining a bath of molten metal, to metal-coat the steel strip,wherein while the steel strip traveling from the annealing furnace tothe molten metal bath passes through a space defined by a snout that islocated on a steel strip delivery side of the annealing furnace and hasan end immersed in the molten metal bath, oxidizing gas is supplied intothe snout, a temperature of an inner wall surface of the snout ismaintained at 150° C. or less below a temperature of the molten metalbath, and an atmospheric temperature of an upper portion in the snout ismaintained at 100° C. or less below the temperature of the molten metalbath.
 2. The continuous hot-dip metal coating method according to claim1, wherein the oxidizing gas comprises any one of nitrogen gascontaining water vapor and nitrogen-hydrogen mixed gas containing watervapor.
 3. The continuous hot-dip metal coating method according to claim1, wherein oxidizability of the oxidizing gas is changed depending on anoperation condition.
 4. The continuous hot-dip metal coating methodaccording to claim 2, wherein an amount of the water vapor in theoxidizing gas is changed depending on an operation condition.
 5. Thecontinuous hot-dip metal coating method according to claim 2, furthercomprising preliminarily investigating, for each operation condition, arelationship between a dew point in the snout and an amount of a defectcaused by non-coating of the steel strip metal-coated under theoperation condition, to determine a target dew point in the snout underthe operation condition, wherein an amount of the water vapor in theoxidizing gas is determined based on the target dew point determined forthe each operation condition.
 6. The continuous hot-dip metal coatingmethod according to claim 5, wherein when the operation condition isswitched over, the amount of the water vapor in the oxidizing gas ischanged based on the target dew point corresponding to the changedoperation condition.
 7. The continuous hot-dip metal coating methodaccording to claim 3, wherein the operation condition is at least one ofa chemical composition of the steel strip, an annealing condition in theannealing, and a component of the molten metal bath.
 8. The continuoushot-dip metal coating method according to claim 3, wherein the operationcondition is a chemical composition of the steel strip.
 9. Thecontinuous hot-dip metal coating method according to claim 1, whereinthe oxidizing gas is supplied from both edges of the snout in atransverse direction of the steel strip.
 10. A continuous hot-dip metalcoating line comprising: an annealing furnace that continuously annealsa steel strip; a coating tank containing a bath of molten metal; a snoutlocated on a steel strip delivery side of the annealing furnace, havingan end immersed in the molten metal bath, and defining a space throughwhich the steel strip continuously supplied from the annealing furnaceinto the molten metal bath passes; a heating unit provided on an outerwall of the snout and in an upper portion in the snout; a gas supplymechanism connected to the snout; and a controller that controls theheating unit and the gas supply mechanism to supply oxidizing gas intothe snout, maintain a temperature of an inner wall surface of the snoutas 150° C. or less below a temperature of the molten metal bath, andmaintain an atmospheric temperature of an upper portion in the snout at100° C. or less below the temperature of the molten metal bath.
 11. Thecontinuous hot-dip metal coating method according to claim 4, whereinthe operation condition is at least one of a chemical composition of thesteel strip, an annealing condition in the annealing, and a component ofthe molten metal bath.
 12. The continuous hot-dip metal coating methodaccording to claim 5, wherein the operation condition is at least one ofa chemical composition of the steel strip, an annealing condition in theannealing, and a component of the molten metal bath.
 13. The continuoushot-dip metal coating method according to claim 4, wherein the operationcondition is a chemical composition of the steel strip.
 14. Thecontinuous hot-dip metal coating method according to claim 5, whereinthe operation condition is a chemical composition of the steel strip.